In recent years, interior permanent magnet (IPM) wind turbine generators have become more and more subject of intensive investigations.
Compared to surface mount permanent magnet (PM) wind turbine generators, a significant advantage of IPM wind turbine generators is that IPM wind turbine generators enable a higher power density. However, an interior permanent magnet (IPM) machine has a salient magnetic rotor. As a result, the electromagnetic power generated by this machine can be categorised into two types. The magnet field power (denoted as FP) is generated by the interaction of magnet field and the stator flux or current perpendicular to it. The reluctance power (denoted as RP) is generated by the interaction of the stator flux or current which aligns with the magnet field and the stator flux or current which is perpendicular to the magnet field. The reluctance power is proportional to the difference of inductance in the ‘d’ and ‘q’ axis, i.e. Ld and Lq. On one hand reluctance power enhances power density of the machine but on the other hand, it is a non-linear quantity that makes the IPM machine become non-linear control plant. It is therefore difficult to construct a linear power control system to ensure predictable dynamic response and system stability at all operating conditions.
In steady state operation; it is desired that IPM controller operates at minimal copper loss (MCL) constraint at low speed and operates within voltage limiting (VL) constraint at high speed. With the increase of IPM generator operating speed, the stator voltage is increased accordingly. The generator speed at which field weakening operation starts at high power condition has been termed as the partial field weakening speed. Above partial field weakening speed, the machine side converter voltage has to be limited within the linear PWM modulation range to minimize the stator current harmonics by applying demagnetization current to the generator. In the field weakening operation, the machine side converter works at closer to unity PWM modulation index value. Moreover, parameter variation due to saturation and temperature change may cause significant deterioration of IPM steady state performance.
Intensive research effort in IPM control has been devoted on achieving MCL control at low speed and VL based field weakening control at high speed to improve the steady state operation efficiency. Most of the published control solutions have not considered the IPM non-linearity. For this type of control system, the controller dynamic performance is unpredictable. The stability margin of IPM control system can not be ensured and the dynamic performance of the IPM machine highly depends on the operation conditions. The steady state performance may deteriorate with generator saturation and temperature changes. Literature suggests the use of 2-dimensional curve fitting approaches. Switching between the 2-dimensional look-up tables or polynomial functions is decided by the operation speed which leads to sudden change of the current reference signals. This results in stability issues in power control system.
The objective of the present invention is to provide a solution to mitigate above-mentioned problems.